With recent development of portable devices, there has been a strong demand for higher-capacity secondary batteries for use in such devices. A nickel hydroxide powder as a cathode material for alkaline secondary battery has also been improved by dissolving cobalt as a solid solution therein to improve a high-temperature utilization ratio or by dissolving zinc or magnesium as a solid solution therein to improve lifetime characteristics.
Further, alkaline secondary batteries have come to be used as high-output power sources such as power sources for hybrid cars, and therefore there has been a strong demand not only for such an improvement in a high-temperature utilization ratio or lifetime characteristics but also for an improvement in output characteristics. However, a nickel hydroxide powder as a cathode material for alkaline secondary battery is an electrical insulating material and poor in conductivity, and therefore has a problem that electric current is not sufficiently distributed to nickel hydroxide and therefore the electrochemical availability of nickel hydroxide is low.
In order to solve such a problem, a cobalt compound such as cobalt oxide or cobalt hydroxide is added as a conductive material to ensure conductivity between nickel hydroxide particles. Such a cobalt compound added is dissolved in a high-concentration alkali metal hydroxide solution used as an electrolytic solution in an alkaline secondary battery, and is oxidized to cobalt oxyhydroxide during electrical charge so that the cobalt oxyhydroxide is deposited on the surface of nickel hydroxide particles. As a result, electrical conductivity is developed and a conductive network is formed between the nickel hydroxide particles.
A cathode using a nickel hydroxide powder and such a cobalt compound as an additive is generally produced by filling the pores of a three-dimensional metal porous body such as a foamed metal (made of nickel metal) with a paste obtained by mixing a nickel hydroxide powder and a cobalt compound powder together with a binder and then subjecting the three-dimensional metal porous body to drying and pressing. However, the cobalt compound powder mixed together with the binder is not necessarily dispersed well in the nickel hydroxide powder. Therefore, such a cathode has a problem that its utilization ratio is significantly lowered under the conditions of use during high-load electrical charge.
As a means for solving such a problem, a method for coating the surface of a nickel hydroxide powder with a cobalt compound has been proposed. For example, Japanese Patent Application Laid-Open (JP-A) No. 63-152866 proposes a nickel active material for storage battery mainly composed of nickel hydroxide particles having a β-type cobalt hydroxide thin layer formed thereon. JP-A No. 63-152866 describes that such a nickel active material can be obtained by depositing a nickel hydroxide powder from a nickel salt in an aqueous alkali solution, and then immersing the nickel hydroxide powder in an aqueous solution of cobalt sulfate salt or cobalt nitrate salt, and then neutralizing the aqueous solution with an aqueous alkali solution.
Further, as a method for producing a cobalt hydroxide-coated nickel hydroxide powder, JP-A No. 7-133115 describes a method in which a cobalt-containing aqueous solution and an ammonium ion supplier are simultaneously, continuously, and quantitatively supplied to a nickel hydroxide powder-containing aqueous solution adjusted to pH 11 to 13 by a caustic alkali.
Further, JP-A No. 2000-149941 proposes a method in which a cobalt ion-containing aqueous solution is supplied to a suspension of a raw nickel hydroxide powder at a supply rate of 0.7 g/min or less in terms of cobalt per kilogram of the raw nickel hydroxide powder and an ammonium ion-containing aqueous solution is supplied to the suspension to achieve a nickel ion concentration of 10 to 50 mg/L and a cobalt ion concentration of 5 to 40 mg/L while the pH, temperature, and ammonium ion concentration of the suspension are kept at predetermined values.
All the methods disclosed in JP-A Nos. 63-152866, 7-133115, and 2000-149941 are intended to ensure the dispersibility and uniformity of a conductive cobalt compound by previously coating the surface of nickel hydroxide powder particles with cobalt hydroxide. However, such conventional methods have a problem that a cobalt hydroxide coating layer is non-uniformly formed on the surface of nickel hydroxide particles or the coating layer is peeled off in the process of producing a paste and therefore it is difficult to ensure the uniformity of a conductive cobalt compound.